The present invention relates to limiters for protection against fault currents in electrical distribution networks, and particularly to limiters having indicators for revealing that a fault current has vaporized a fusible element of the limiter.
The chief purposes of network protection are to interrupt fault currents before damage occurs to cable insulations and associated equipment, and to eliminate service interruptions. There are two principal kinds of fault currents: high energy arcing faults and sustained faults. The arcing fault is more common and can sputter briefly before clearing. The arcing fault can also be of such duration as to "roast" the insulation. A sustained fault occurs when two conductors contact solidly with each other and induce the flow of heavy short circuit currents. Suitable protection in the form of limiters is required to prevent damage to cable insulation, combustible fumes, fire and explosion caused by fault currents. Accordingly limiters are installed at each end of cable sections and are selected according to specific time-current characteristics to avoid unnecessary outages. In this way limiters act to confine fault current damage to the section of cable where the fault occurs.
When limiters are installed in electric distribution networks, particularly underground networks, the limiter element and cable ends are securely covered. The insulating covering, with joints wrapped in tape, prevents easy identification of a particular limiter which has operated, among many in the distribution network. The task of finding a limiter which has operated or fused involves the tedious and time consuming task of uncovering and recovering a series of limiter assemblies.
Many techniques for blown fuse indication have been developed and include spring loaded indicators, visual windows, color changes and pressure dependent indication. All pressure indicators thus far have been destructive, single use operating devices. Expulsion fuses discharge a weighted indicator designed to remove energy from the circuit and relieve internal pressure. The total fuse must be replaced. U.S. Pat. No. 4,016,521 to Seybold discloses an overpressure limiter indicator including a plastic wall which melts and is forced out by internal pressure caused by limiter fusing. The Seybold device allows a single use because the unit including fusible link and housing is permanently altered to indicate limiter operation and must be removed and replaced for the circuit to be reset.